Core Java Interview Questions -26
1:Can an application have multiple classes having main method?
Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for
the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.
2:Can I have multiple main methods in the same class?
No the program fails to compile. The compiler says that the main method is already defined in the class.
3:What if the main method is declared as private?
The program compiles properly but at runtime it will give “Main method not public.” message.
4:What if the static modifier is removed from the signature of the main method?
Program compiles. But at runtime throws an error “NoSuchMethodError”.
5:What is the first argument of the String array in main method?
The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.
6:If I do not provide any arguments on the commandline, then the String array of Main method will be empty or null?
It is empty. But not null.
7:How can one prove that the array is not null but empty using one line of code?
Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a
NullPointerException on attempting to print args.length.
8:What if I write static public void instead of public static void?
Program compiles and runs properly.
9:What if I do not provide the String array as the argument to the method?
Program compiles but throws a runtime error “NoSuchMethodError”.
10:What environment variables do I need to set on my machine in order to be able to run Java programs?
CLASSPATH and PATH are the two variables.
11:What are Checked and UnChecked Exception?
A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses.
Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown.
eg, IOException thrown by java.io.FileInputStream’s read() method·Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the
compiler doesn’t force client programmers either to catch theexception or declare it in a throws clause. In fact, client programmers may not even know that the exception could
be thrown. eg, StringIndexOutOfBoundsException thrown by String’s charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.
12:What is Overriding?
When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass.When the method is invoked for an object of the class, it is the new definition of the method that is called, and not
the method definition from superclass. Methods may be overridden to be more public, not more private.
13:Do I need to import java.lang package any time? Why ?
No. It is by default loaded internally by the JVM.
14:Can I import same package/class twice? Will the JVM load the package twice at runtime?
One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the
JVM will internally load the class only once no matter how many times you import the same class.
15:What is the difference between an Interface and an Abstract class?
An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods,
but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class
members (private, protected, etc.), but has some abstract methods.
16:What is the purpose of garbage collection in Java, and when is it used?
The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes
unreachable to the program in which it is used.
17:Describe synchronization in respect to multithreading.
With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable
while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.
18:Explain different way of using thread?
The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, ’cause
when you are going for multiple inheritance..the only interface can help.
19:What are pass by reference and passby value?
Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be
20:What is HashMap and Map?
Map is Interface and Hashmap is class that implements that.
21:Difference between HashMap and HashTable?
The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and
value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is
unsynchronized and Hashtable is synchronized.
22:Difference between Vector and ArrayList?
Vector is synchronized whereas arraylist is not.
23:Difference between Swing and Awt?
AWT are heavy-weight componenets. Swings are light-weight components.
24:What is the difference between a constructor and a method?
A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type,
and is invoked using the new operator.A method is an ordinary member function of a class. It has its own name, a
return type (which may be void), and is invoked using the dot operator.
25:What is an Iterator?
Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a
collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection
itself while traversing an Iterator.
26:State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package
relationships on declared items qualified by these modifiers.
public : Public class is visible in other packages, field is visible everywhere (class must be public too)
private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may
only be accessed by the class that owns the feature.
protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This
access is provided even to subclasses that reside in a different package from the class that owns the protected feature.
default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular
27:What is an abstract class?
Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not
call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared
as such.A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.
28:What is static in java?
Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without
creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be
shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can’t override a static method
with a nonstatic method. In other words, you can’t change a static method into an instance method in a subclass.
29:What is final?
A final class can’t be extended ie., final class may not be subclassed. A final method can’t be overridden when its class is inherited. You can’t
change value of a final variable (is a constant).
30:Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?
Yes the imports are checked for the semantic validity at compile time. The code containing
above line of import will not compile. It will throw an error saying,can not resolve symbol
symbol : class ABCD
location: package io
31:Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?
No you will have to import the subpackages
explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it’s subpackage.
32:What is the difference between declaring a variable and defining a variable?
In declaration we just mention the type of the variable and it’s name. We do not initialize it. But defining means declaration + initialization.
e.g String s; is just a declaration while String s = new String (“abcd”); Or String s = “abcd”; are
33:What is the default value of an object reference declared as an instance variable?
null unless we define it explicitly.
34:Can a top level class be private or protected?
No. A top level class can not be private or protected. It can have either “public” or no modifier.
supposed to have a default access.If a top level class is declared as private the compiler will
complain that the “modifier private is not allowed here”. This means that a top level class can not be private. Same is the case with
35:What type of parameter passing does Java support?
In Java the arguments are always passed by value.
36:Primitive data types are passed by reference or pass by value?
Primitive data types are passed by value.
37:Objects are passed by value or by reference?
Java only supports pass by value. With objects,the object reference itself is passed by value
and so both the original reference and parameter copy both refer to the same object .
38:What is serialization?
Serialization is a mechanism by which you can save the state of an object by converting it to a
39:How do I serialize an object to a file?
The class whose instances are to be serialized should implement an interface Serializable.
Then you pass the instance to the
40:Which methods of Serializable interface should I implement?
The serializable interface is an empty interface,it does not contain any methods. So we do not
implement any methods.
41:How can I customize the seralization process? i.e. how can one have a control over the serialization process?
Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This
interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic
for customizing the serialization process.
42:What is the common usage of serialization?
Whenever an object is to be sent over the network, objects need to be serialized.Moreover if the state of an object is to be
saved, objects need to be serilazed.
43:What is Externalizable interface?
A:Externalizable is an interface which contains two methods readExternal and writeExternal.These methods give you a control over the
serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.
44:When you serialize an object, what happens to the object references included in the object?
The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object
references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also
serialized alongwith the original obect.
45:What one should take care of while serializing the object?
One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.
46:What happens to the static fields of a class during serialization?
There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are
1. Serialization ignores static fields, because they are not part of ay particular state state.
2. Base class fields are only hendled if the base class itself is serializable.
3. Transient fields.
47:Does Java provide any construct to find out the size of an object?
No there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java.
48:Give a simplest way to find out the time a method takes for execution without using any profiling tool?
Read the system time just before the method is invoked and immediately after method returns. Take
the time difference, which will give you the time taken by a method for execution.
To put it in code…
long start = System.currentTimeMillis ();
long end = System.currentTimeMillis ();
System.out.println (“Time taken for execution is ” + (end – start));
Remember that if the time taken for execution is too small, it might show that it is taking zero milliseconds
for execution. Try it on a method which is big enough, in the sense the one which is doing considerable
amout of processing.
49:What are wrapper classes?
Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character,
50:Why do we need wrapper classes?
It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store
objects and not primitive data types. And also the wrapper classes provide many utility methods also.
Because of these resons we need wrapper classes. And since we create instances of these classes we can
store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method
expects an object.
51:What are checked exceptions?
Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked
52:What are runtime exceptions?
Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data
or because of wrong business logic etc. These are not checked by the compiler at compile time.
53:What is the difference between error and exception?
An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM
errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad
input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a
null reference. In most of the cases it is possible to recover from an exception (probably by giving user a
feedback for entering proper values etc.).
54:How to create custom exceptions?
Your class should extend class Exception, or some more specific type thereof.
55:If I want an object of my class to be thrown as an exception object, what should I do?
The class should extend from Exception class. Or you can extend your class from some more precise
exception type also.
56:If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?
One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well.
57:How does an exception permeate through the code?
An unhandled exception moves up the method stack in search of a matching When an exception is thrown
from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found
then that block will be invoked. If a matching type is not found then the exception moves up the method
stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If
it does not find such a block then finally the program terminates.
58:What are the different ways to handle exceptions?
There are two ways to handle exceptions,
1.By wrapping the desired code in a try block followed by a catch block to catch the exceptions.
2.List the desired exceptions in the throws clause of the method and let the caller of the method hadle those exceptions.
59:What is the basic difference between the 2 approaches to exception handling.1> try catch block and2> specifying the candidate exceptions in the throws clause?When should you use which approach?
In the first approach as a programmer of the method,
you urself are dealing with the exception. This is fine if you are in a best position to decide should be done
in case of an exception. Whereas if it is not the responsibility of the method to deal with it’s own exceptions, then do not use this approach. In this
case use the second approach. In the second approach we are forcing the caller of the method to
catch the exceptions, that the method is likely to throw. This is often the approach library creators use.
They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.
60:Is it necessary that each try block must be followed by a catch block?
It is not necessary that each try block must be followed by a catch block. It should be followed by
either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.
61:If I write return at the end of the try block, will the finally block still execute?
Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return.
62:If I write System.exit (0); at the end of the try block, will the finally block still execute?
No in this case the finally block will not execute because when you say System.exit (0); the control
immediately goes out of the program, and thus finally never executes.
63.What is singleton class?where is it used?
Singleton is a design pattern meant to provide one and only one instance of an object.
Other objects can get a reference to this instance through a static method (class constructor
is kept private). Why do we need one? Sometimes it is necessary, and often sufficient, to
create a single instance of a given class. This has advantages in memory management, and
for Java, in garbage collection. Moreover, restricting the number of instances may be
necessary or desirable for technological or business reasons–for example, we may only
want a single instance of a pool of database connections.
64.What is a compilation unit?
The smallest unit of source code that can be compiled, i.e. a .java file.
65.Is string a wrapper class?
String is a class, but not a wrapper class. Wrapper classes like (Integer) exist for each
primitive type. They can be used to convert a primitive data value into an object, and viceversa.
66.What is serialization?
Quite simply, object serialization provides a program the ability to read or write a whole
object to and from a raw byte stream. It allows Java objects and primitives to be encoded
into a byte stream suitable for streaming to some type of network or to a file-system, or
more generally, to a transmission medium or storage facility. A seralizable object must
implement the Serilizable interface. We use ObjectOutputStream to write this object to a
stream and ObjectInputStream to read it from the stream.
67.Why there are some null interface in java ? What does itmean ? Give me some null interfaces in JAVA?
Null interfaces act as markers..they just tell the compiler that the objects of this class need
to be treated differently..some marker interfaces are : Serializable, Remote, Cloneable
68.What is a transient variable?
A transient variable is a variable that may not be serialized.
69.Is synchronised a modifier?indentifier??what is it??
It’s a modifier. Synchronized methods are methods that are used to control access to an
object. A thread only executes a synchronized method after it has acquired the lock for the
method’s object or class. Synchronized statements are similar to synchronized methods. A
synchronized statement can only be executed after a thread has acquired the lock for the
object or class referenced in the synchronized statement.
70.Is Iterator a Class or Interface? What is its use?
Iterator is an interface which is used to step through the elements of a Collection.
71.What is similarities/difference between an Abstract class and Interface?
Differences are as follows:
• Interfaces provide a form of multiple inheritance. A class can extend only one other
• Interfaces are limited to public methods and constants with no implementation.
Abstract classes can have a partial implementation, protected parts, static methods,
• A Class may implement several interfaces. But in case of abstract class, a class may
extend only one abstract class.
• Interfaces are slow as it requires extra indirection to to find corresponding method
in in the actual class. Abstract classes are fast.
• Neither Abstract classes or Interface can be instantiated.
72.Why java is not a 100% oops?
Many people say this because Java uses primitive types such as int, char, double. But then
all the rest are objects. Confusing question..
73.What is a resource bundle?
In its simplest form, a resource bundle is represented by a text file containing keys and a
text value for each key.
74.What is transient variable?
Transient variable can’t be serialize. For example if a variable is declared as transient in a
Serializable class and the class is written to an ObjectStream, the value of the variable can’t
be written to the stream instead when the class is retrieved from the ObjectStream the
value of the variable becomes null.
75.Which containers use a border Layout as their defaultlayout?
The window, Frame and Dialog classes use a border layout as their default layout.
76.Why do threads block on I/O?
Threads block on i/o (that is enters the waiting state) so that other threads may execute
while the i/o Operation is performed.
77.How are Observer and Observable used?
Objects that subclass the Observable class maintain a list of observers. When an Observable
object is updated it invokes the update() method of each of its observers to notify theobservers that it has changed state. The Observer interface is implemented by objects that
observe Observable objects.
78. What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to control the access of
multiple threads to shared resources. Without synchronization, it is possible for one thread
to modify a shared object while another thread is in the process of using or updating that
object’s value. This often leads to significant errors.
79.Can a lock be acquired on a class?
Yes, a lock can be acquired on a class. This lock is acquired on the class’s Class object..
80.What’s new with the stop(), suspend() and resume() methods in JDK 1.2?
The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.
81.Is null a keyword?
The null value is not a keyword.
82. What is the preferred size of a component?
The preferred size of a component is the minimum component size that will allow the
component to display normally.
83.What method is used to specify a container’s layout?
The setLayout() method is used to specify a container’s layout.
84.Which containers use a FlowLayout as their default layout?
The Panel and Applet classes use the FlowLayout as their default layout.
85.What state does a thread enter when it terminates its processing?
When a thread terminates its processing, it enters the dead state.
86.What is the Collections API?
The Collections API is a set of classes and interfaces that support operations on collections
87.Which characters may be used as the second character of an identifier, but not as the first character of an identifier?
The digits 0 through 9 may not be used as the first character of an identifier but they may
be used after the first character of an identifier.
88.What is the List interface?
The List interface provides support for ordered collections of objects.
89.How does Java handle integer overflows and underflows?
It uses those low order bytes of the result that can fit into the size of the type allowed by
90.What is the Vector class?
The Vector class provides the capability to implement a growable array of objects
91.What modifiers may be used with an inner class that is a member of an outer class?
A (non-local) inner class may be declared as public, protected, private, static, final, or
92.What is an Iterator interface?
The Iterator interface is used to step through the elements of a Collection.
93.What is the difference between the >> and >>>operators?
The operator carries the sign bit when shifting right. The >>> zero-fills bits that have
been shifted out.
94.How many bits are used to represent Unicode, ASCII,UTF-16, and UTF-8 characters?
Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses
only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and
18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.
95.What is the difference between yielding and sleeping?
When a task invokes its yield() method, it returns to the ready state. When a task invokes
its sleep() method, it returns to the waiting state.
96. Which java.util classes and interfaces support event handling?
The EventObject class and the EventListener interface support event processing.
97.Is sizeof a keyword?
The sizeof operator is not a keyword.
98.What are wrapped classes?
Wrapped classes are classes that allow primitive types to be accessed as objects.
99. Does garbage collection guarantee that a program willnot run out of memory?
Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection
100.What restrictions are placed on the location of apackage statement within a source code file?
A package statement must appear as the first line in a source code file (excluding blank
lines and comments).
101.What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the waiting or
dead states or a higher priority task comes into existence. Under time slicing, a task
executes for a predefined slice of time and then reenters the pool of ready tasks. The
scheduler then determines which task should execute next, based on priority and other